1. Field of the Invention
The invention relates to a receiver for a digital transmission system, in which transmitted data contain a training data sequence from which a channel impulse response for the transmit channel is estimated in a channel estimator with the aid of a training data sequence stored in the receiver.
2. Description of the Related Art
Receivers of such a type will be used, for example, in the future Pan-European mobile radio system. In this mobile radio system speech signals are co-transmitted in digitized form with other digital signals according to a time-division multiple-access method. A transmitter of the mobile radio system transmits these data which are appropriately modulated. As a result of reflections and multipath propagation on the transmission path, the transmitted signal reaches the receiver in various superposed signal portions which have different delays and phase shifts. This phenomenon distorts the transmitted signal. These distortions cause the data symbols contained in the received signal to be influenced by preceding data symbols (intersymbol interference).
To detect the originally transmitted data symbols it is necessary to eliminate these distortions by means of equalization. For this purpose a channel characteristic that describes the transmission path may be provided as a predetermined value to an adaptive equalizer. However, the transmission path between transmitter and receiver and consequently, also the channel information describing each transmission path, are changed continuously as a result of changes of location of the mobile radio receiver. Therefore, this channel characteristic is re-determined each time by means of a channel measurement to be performed for each time slot. For this purpose, a training data sequence having constant data contents is used, with the data contents of the training data sequence being stored in each receiver. This training data sequence is embedded in each transmitted data sequence. A channel estimator arranged in each receiver estimates a channel impulse response by processing the training data sequence received in a distorted manner and the training data sequence stored in undistorted form, which response is applied to the equalizer.
The measurement of the quality of the received signal is a major factor in the estimation of the received signal. For estimating this receiving quality there are possibilities especially known from analog circuit designs. For example, the signal strength (field strength measurement) or a signal-to-noise ratio may be selected. In DE-OS 34 27 473 to which U.S. Pat. No. 4,578,819 corresponds, for receiving space-division transmitted analog radio signals, in which the signal-to-noise ratio is measured in a manner so that the demodulated audio signal is applied to a high-pass filter. The amplitude ratio of the high-pass filtered analog signal (noise components) to that of the unfiltered analog signal provides a voltage that approximately corresponds to the signal-to-noise ratio.
Analog measuring methods of this type are suitable for evaluating a digital receiver only to a limited extent, for example, due to the large frequency bandwidth of a digital signal.
For estimating the receiving quality of input signals to be processed digitally, the measurement of the bit error rate of the detected data symbols would particularly correspond to the selected signal processing. However, this is very costly.